1. Field of the Invention
The present invention relates to a control unit for a variable valve timing mechanism (VTC), in which the rotation phase of a camshaft with respect to a crankshaft is changed to change at least either valve timing (opening and closing timing) of an intake valve or an exhaust valve, and a control method for the same.
2. Description of Related Art
To make valve timing freely changeable, an electrically operated VTC is known in which the rotation phase of the camshaft with respect to the crankshaft is changed by an electric motor as described in Japanese Laid-Open (Kokai) Patent Application Publication No. 2008-57371.
VTC attached to an end of the camshaft is subjected to an alternating torque (cam torque), made up of a reaction force of a valve spring and an inertial force by the mass of a movable part, according to the rotation of the camshaft. For example, in a low rpm (revolutions per minute) range of the engine, in which the alternating frequency of the cam torque is equal to or less than the drive frequency of VTC, there is a possibility that a torque necessary to change the valve timing is greater than the maximum torque of the motor unlike in a high rpm range of the engine, in which the cam torque can be regarded as an average value. In this case, the motor enters into a “locked state” in which the rotation of the motor is stopped. Since not only is no back electromotive force is generated, but also the VTC angle does not reach a target angle, an excess current (lock current) is generated in order to approximate the target angle. When the motor enters into the locked state, the motor torque is not used to change the valve timing, resulting in an increase in power consumption due to the lock current.